Vehicles today use, for example, urea as reducing agent in SCR (selective catalytic reduction) systems which comprise an SCR catalyst in which said reducing agent and NOx gas can react and be converted to nitrogen gas and water. Various types of reducing agents may be used in SCR systems. AdBlue is an example of a commonly used reducing agent.
One type of SCR system comprises a container which holds a reducing agent. The system has also a pump adapted to drawing said reducing agent from the container via a suction hose and supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is adapted to injecting a necessary amount of reducing agent into the exhaust pipe upstream of the SCR catalyst according to operating routines which are stored in a control unit of the vehicle. To make it easier to regulate the pressure when there are small or no dosing amounts, the system comprises also a return hose which runs back to the container from a pressure side of the system.
The SCR catalyst in the vehicle's exhaust duct comprises inter alia a vaporisation module and a portion with an SCR substrate. Said vaporisation portion is adapted to vaporising dosed reducing agent in order to achieve better mixing between the exhaust gases and said dosed reducing agent.
In cases where not all of the dosed reducing agent is vaporised, precipitates of it may form, normally consisting of crystals which attach themselves to one or more parts of the vaporisation module. With continued dosing of reducing agent these crystals may build up further and are likely to lead to undesirable consequences. The problem of the build-up of reducing agent crystals in SCR systems of motor vehicles is well known.
One cause of said undesirable build-up of reducing agent crystals may be that the vaporisation capacity of the vaporisation module in certain operating situations is overestimated. Said reducing agent crystals may also be referred to as urea stones.
There are a number of problems associated with build-up of urea stones in the SCR catalyst's vaporisation module.
Firstly, an exhaust backpressure may increase in the engine's exhaust system, imposing a greater load upon the engine and thereby forcing it to work unnecessarily hard, at undesirably high load.
Secondly, a degree of conversion by the SCR catalyst may be impaired, causing an increase in undesirable emissions from the vehicle.
Thirdly, in cases where build-up of urea stones is not prevented the exhaust system may eventually become totally obstructed, completely preventing the passing of an exhaust flow.